Chinese scientists announced new results from surveys conducted at the landing site of the Zurong rover on Mars, in the plain of Utopia Planitia, in the Northern Hemisphere.
The team revealed that they identified irregular polygonal structures located at a depth of about 35 meters throughout the robot's journey.
The structures measure from centimeters to tens of metres. Scientists believe the buried polygonal terrain resulted from freeze-thaw cycles on Mars billions of years ago, but it could also be volcanic, the result of cold lava flows.
The Zhurong spacecraft landed on Mars on May 15, 2021, making China the second country ever to successfully land on Mars.
The rover, named after the Chinese god of fire, explored its landing site and sent back images - including a selfie with the lander, taken by a remote camera - to study the Martian terrain and make measurements using a ground-penetrating radar (GPR) instrument.
Zurong's initial mission life was three Earth months, but it operated successfully for just over one Earth year before entering a planned hibernation state. However, no news has been heard from the vehicle since May 2022.
Scientists from the Institute of Geology and Geophysics of the Chinese Academy of Sciences who worked with Zurong's data said the GPR instrument provides an important complement to orbital radar explorations from missions such as the European Space Agency's Mars Express and China's Tianwen-1 orbiter. Pointing out that an in-situ GPR survey can provide important local details about shallow structures and their composition at a depth of about 100 meters along the vehicle’s path.
The Utopia Planitia region is known as a large plain within Utopia, the largest recognized impact basin on Mars (and also in the solar system) with an estimated diameter of 3,300 km.
In total, the vehicle traveled a distance of 1,921 meters during its lifespan.
The team, led by Li Zhang, wrote in their paper published in the journal Nature Astronomy that the spacecraft's radar detected sixteen polygonal structures at a distance of about 1.2 kilometers, indicating a wide distribution of similar terrain beneath "Utopia Planitia."
These discovered features were likely formed 3.7 to 2.9 billion years ago during the Late Hesperian and Early Amazonian eras on Mars, “possibly with the cessation of the ancient wet environment. The ancient polygonal terrain was later buried, with or without erosion,” by Subsequent geological processes.
Although polygonal terrain has been seen across several regions of Mars by several previous missions, this is the first time there have been indications of buried polygonal features.
The team wrote that the buried polygonal terrain required a cold environment, which may be related to the freezing and thawing of water and ice in southern Utopia Planitia, between 3.7 and 2 billion years ago.
Previous research from Zorong's radar data indicated that multiple floods during the same time frame created several layers beneath the surface of Utopia Planitia.
Likewise, they explained that the most likely formation mechanisms could be due to shrinkage due to drying of wet sediments that produces clay cracks, as well as thermal shrinkage caused by cooling of lava, which also produces cracks.
Hypotheses about the origin of the discovered polygonal structures make experts believe that there was a climate change in the low to medium latitudes of Mars due to the great tilt of the planet.
The Chinese team explained, “The subsurface structure with materials covering the buried ancient polygonal terrain indicates that there was a marked shift in paleoclimate at some point after that. The contrast above and below a depth of about 35 meters represents a marked shift in water activity or thermal conditions in "The time of ancient Mars, which means there was climate disturbance at low to mid latitudes."